<p>Capnometry measures end-tidal carbon dioxide (P_EtCO₂) in expired air and is standard practice for monitoring mechanical ventilation in pediatric critical care. P_EtCO₂ is often used as a non-invasive indicator for arterial CO₂ (P_aCO₂), while under normal conditions the P_aCO₂-P_EtCO₂ difference is 2–5 mmHg due to imperfect matching of ventilation-perfusion (V/Q). However, in some pediatric patients, a reversed (negative) P(a-Et)CO₂ gradient (ventilatory gap) can be observed, where P_EtCO₂ exceeds P_aCO₂. While this phenomenon is commonly linked to underlying technicalities, physiological explanations, including relatively high closing volumes in infants, poorly compliant lung tissue, and elevated mixed venous CO₂ (Pv̅CO₂) due to increased CO₂ production during critical illness, have also been described in the literature. However, current evidence for these mechanisms is limited, and further research is needed to clarify their roles. Possibly, awareness of both technical and physiological factors that may underlie a negative P(a–Et)CO₂ gradient can help clinicians interpret observations more cautiously and consider appropriate adjustments to mechanical ventilation strategies as needed.</p>